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What is the typcial descent rate of a helicopter?
I'm not a pilot, just asking out of curiosity. What is the typical descent rate for a helicopter when landing? I found the answer of 300 feet per minute on the web, but I want to confirm this. If this is true, than a helicopter descending from 2400' to land would take 8 minutes. Is this correct?

That is of course dependent on airspeed at approach, and the approach angle. I have a flight manual of an older type helicopter, I cannot reproduce the contents, but it shows True Airspeed on the horizontal axis, Rate of Descent on the vertical, and a set of inclined lines for the angle of Descent.
In the Emergency Procedures - Normal Approach, an Angle of Descent of 8 - 10º is specified, and a typical TAS of 65 knots. This amounts to a Rate of Descent between 800 and 1100 ft/min. Maximum allowable ROD at that speed is 1600 ft/min which is marked as Approximate Autorotation.

Thanks. Doesn't "Emergency Procedures" imply higher than typical rate of descent? This would suggest that normal rate of descent is lower.

Yes it would, although use of Normal Approach would suggest, well, normalcy. The normal angle of descent seems to be 8 - 10 deg for this helicopter, a Huey. Could not find any other references to a normal descent rate in it.

It also depends upon the exact aircraft make/ model/ equipment as a "normal descent" can put you in the bad area of the height-velocity diagram for potentially a long period of time in certain circumstances. Managing the risk of that versus a descent that is not particularly stabilized is the art of flying. In the particular helicopter I had trained in, with a stabilized approach from about 50' AGL to 6' AGL you were in the bad area of the H-V diagram and if the engine fails it's really going to hurt.

The 300 feet per minute rate is the maximum descent rate for a hover. In helicopters, it is the rotor that is providing airflow downwards to keep the helicopter in the air. The air that has traveled through the rotor, downwards, keeping the helicopter aloft has become very turbulent air with a downward velocity. This means that it until it settles, it is not going to provide lift. If I'm in a hover, and I descend at a rate higher than 300fpm, then I can potentially descend into my own down-wash, and then the rotor stops providing lift. This is known as vortex ring state, or sometimes settling with power (Vortex Ring State is more precise, settling with power can occur for various reasons and in different conditions). If I'm traveling in some direction at a reasonable speed, I'll be moving away from the turbulent air that just went through the rotor, and the low descent speed no longer applies.
The calculations from the earlier post are reasonably accurate for an emergency descent. In an emergency landing (an autorotation, or power-off landing), the R22 emergency procedures state to establish your glide at 65 knots, with your vertical descent being controlled by what is required for me to keep the rotor spinning.
In a normal descent, my forward airspeed would be closer to 90 knots, and 8-10 degrees would be on the higher end for descent rates.

Thanks. So what does a typical helicopter (e.g. for sightseeing) descending from 2400' do usually? Hover down for 8 minutes or adopt some kind of a spiral path to speed up the descent?

Helicopters use far more power in a hover than they do in forward flight. In forward flight, anything above about 30 knots, the helicopter starts developing translational lift, where the entire spinning rotor (the rotor disc) acts like a wing and generates extra lift. Due to both the translational lift, the restrictions on the descent speed in a hover, and the ability to merge with regular airport traffic, in most situations, helicopters will land in the exact same pattern as an airplane, except that we'll generally circle around in the opposite direction (making right turns instead of left).


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Why is colour of aircraft access panel outlines darker than the rest of the aircraft?
I've seen that in nearly every photo of a dark-coloured aircraft (especially prominent in military jets) its panels have a darker colour compared to the rest of the aircraft.
What is the reason for this?

Probably they are painted separately, with different batch of paint or at different times. Just like cars, matching paint is hard.

I don't mean the gaps themselves, which I'd call panel lines (and when eg scale modelling someone would use maybe oils to make them stand out), I mean the difference in colour, also seen on the F-16's tail.

I can't see what you mean, the panel are light gray because it happens this area of the camouflage pattern is light gray.

What I mean is that in the grey-camouflaged area I am pointing, for example, you can see the panel outlines being darker. Also, if you see the tail, you'll also see areas where the paint looks darker.

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What is the relationship between speed, torque and pitch in a helicopter?
Can someone clarify the relationship between aircraft speed, torque, prop length and pitch and how this applies to a helicopter.
In addition I want to know if the amount of lift a helicopter generates changes dependant on pitch, how much does the pitch change?

Aircraft pitch or rotor pitch?

Rotor pitch Sanchises

I'm afraid that a thorough answer requires the contents of chapters 2 and 3 of Leishman, but here is an outline. The relationship between power and true airspeed from this answer:

Power equals torque times angular velocity of the rotor: $ P = Q * \Omega$. Most helicopters have constant speed rotors, so the relationship between torque and speed would be similar to the power curves.
Rotor blade length and pitch appear as follows. Power P and torque Q equations for momentum theory in the hover:
$$P = C_P \cdot ½ \rho A (\Omega R)^3$$
$$Q = C_Q \cdot ½ \rho A (\Omega R)^2  R$$
At forward speed the momentum based power becomes less due to lower induced power requirements. An inflow equation relates forward velocity of the helicopter with blade tip speed - too much scope for an answer here.
Blade length appears in the rotor disk area A. Torque and power are functions of the rotor solidity $\sigma$, defined as (total blade area)/(disk area). Note that $C_P$ is always the same value as $C_Q$
From Leishman:

..if the amount of lift a helicopter generates changes dependant on pitch, how much does the pitch change..
Thrust T of the rotor Again from Leishman:

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How to know that ADS-B position messages are not spaced more then the 1/2 zone limit?
I am doing some ADS-B compact position reporting (CPR) decoding at the moment and one of the things that is described is;
"1/2 zone limit" from the last successfully decoded message.
I understand the reason behind it but how can we be sure that new message location is in 1/2 zone limit from the last decoded message?
Note : I am talking about local CPR decoding.

Usually you use a time limit between ADS-B position messages depending on the type of encoding used (surface or airborne)  and related assumed maximum speed to ensure that the aircraft cannot have travelled more than half a zone. If you don't receive a new position message within the time limit you will have to use global encoding (for which you need an odd/even pair of position messages within 10 seconds).
If I remember correctly, for airborne decoding an assumed maximum ground speed of 1000 knots is normally used. For surface decoding it is probably 200 knots. The latter gives problems when airborne aircraft send messages in surface format (e.g. due to a system fault).
In addition you can perform a number of tests that show that the newly decoded position is reasonably close to where it was to be expected.
For details see RTCA DO260B or EUROCAE ED102A (they are equivalent). They describe the encoding and decoding of (1090ES) ADS-B messages for airborne applications. In addition you can use EUROCAE ED129B. It is dedicated to ADS-B receiver systems for ATC (for gound based and space based receivers) and it includes a description of the decoding logic and related tests.


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Is the user interface design of ATC equipment generic across countries?
Are the computer screens used by ATC generic across different countries? I.e., do ATCs around the world simply purchase the same software from a few companies, or do each of them develop their own system and end up with different user interfaces?
For an analogy of aircrafts, most of the controls and instruments are universal across pretty much all aircrafts, be it Cessna, Piper, Airbus, Boeing, CRJ etc. Any trained pilot would likely recognize the important controls (e.g. throttle, flap lever, attitude indicator) right away; certain colors and symbols are also expected.
Is this the same for the computer systems used by ATC? E.g. if a controller from the US goes to Australia, is he expected to understand and know how to operate the equipment right away?

Most ATC software systems are developed by a few large companies. Companies that produce such software include Lockheed Martin, Thales, Raytheon, Indra, Comsoft, Adacel (and maybe a few others). It's not a big market since there are only a limited number of customers (countries) and safety critical software is very expensive to develop.
One interesting offering is the Albatross open source ATC software. This isn't a complete system (yet?) but some parts are available now. I'm not sure whether this software is actually used in operations anywhere at this point.
Most ATC providers (which are usually run or owned by their government, though not always) do not do their own software development. They buy a system from one of the established vendors, who may do some amount of customisation work ranging from none to extensive, perhaps with a support contract for future fixes or enhancements. There are a few exceptions though, for example Airways New Zealand uses software originally developed by a large vendor but also has an in-house software development group who maintains, enhances, and extends the existing software.
Each system has the same general operating concept - an ASD (Air Situation Display) which shows the locations of aircraft, plus auxiliary windows that may contain all kinds of other things used for communication and coordination. Like any sophisticated computer software, the general idea of two different systems might appear the same, but be completely different to actually operate (think of Photoshop vs. Gimp, or Microsoft Word vs. Apple Pages).
A qualified controller from the US would not be expected to be able to operate in Australia straight away. Not only will the software be different (different mouse clicks, or different key commands), but also the rules of air traffic control can be different between countries. Furthermore, a controller must become familiar with the local airspace and operating procedures.
Having said that, the general skills of air traffic controllers are portable between countries. On moving from one country to another, a controller would be expected to go through a certain amount of training in their new country, which could range from brief familiarisation to more rigorous retraining.

Like anything it varies from place to place and even within countries. This is mainly because not every ATC facility is built at the same time. You are more likely to find equipment from the same era as the building or the last time it had a major over haul. For what its worth you are correct that there are only a handful of companies making ATC equipment (radar etc.) but through out the course of time the models vary. Some less economically profitable countries may even buy units second hand from others. In the end of the day Radar is Radar and most if not all radar screens will look at least generally similar.
FWIW airplanes have highly varying control designs. While Boeing employs a yoke Airbus is partial to the side stick. Cessna's typically use push/pull throttles that are rod style as well as electric flaps while Piper uses lever throttles and Johnson bar flaps, Older Mooneys even sport johnson bar landing gear... So not all planes are made the same shy of the instruments which are generally the same aside from older Russian attitude indicators which appear reversed from the ones typically found in other planes. I will agree that any pilot should be able to identify the controls they are of highly varying design.

While cockpit designs do vary, some elementary designs are kept the same, so they can be easily identified. For example you won't find the prop lever in green and the mixture in purple.

True I have however seen a white prop lever

And this Ercoupe with a deceiving red throttle lever

Interesting cockpit. But my question is asking more like, in general sense

I understand I was just throwing it out there for the sake of an example. Generally modern stuff is color coded but control layout does vary.


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What will air traffic controllers do when radio contact is lost?
When a pilot loses radio contact with ATC, there are standard procedures to follow, e.g. squawk 7600, observe the traffic pattern, observe the red / green light gun.
On the other side, if a controller attempts to contact an aircraft and gets no response, what will they do? Assume that radar contact is not lost, or the controller is not equipped with radar.

EDIT: Let say a controller spotted a 7600. What's next?

The answer to this is buried somewhere in a related question I can't find!  In essence, providing the aircraft continues as planned, they don't do much other than to make sure nearby traffic is alerted and maintain separation.

@Simon this one? http://aviation.stackexchange.com/q/13677/609

You didn't mention any specific jurisdiction but for the FAA, the expected actions are in the ATC orders. Section 5-2-8 says:

When you observe a  Code 7600  display, apply the procedures in para
  10−4− 4, Communications Failure

Section 10-4-4 gives general instructions for comms failure, it's too long to quote in full but the first paragraph summarizes it:

In the event of lost communications with an aircraft under your
  control jurisdiction use all appropriate means available to
  reestablish communications  with the aircraft. These may include, but
  not be limited to, emergency frequencies, NAVAIDs that are equipped
  with voice  capability, FSS,  Aeronautical Radio Incorporated (ARINC),
  etc.

The instructions suggest some specific actions:

Broadcast via NAVAID frequencies
Ask the aircraft to use its transponder or make turns to acknowledge instructions

If comms haven't been reestablished within 5 minutes, ATC is instructed to consider it as suspicious:

If radio communications have not been (re) established  with the
  aircraft after 5 minutes, consider the aircraft’s or pilot’s activity
  to be suspicious and report it to the FLM/CIC per FAA Order JO 7610.4,
  Chapter 7, Hijacked/Suspicious Aircraft Reporting and Procedures

Section 10-2-5 also says that lost comms are an emergency from the ATC perspective and should be escalated to the ARTCC. If comms aren't restored within 30 minutes then the aircraft is considered overdue, which triggers another set of procedures listed in section 10-3.
As for clearing airspace or whatever, there are no specific instructions in the orders, it just says that:

When an IFR aircraft experiences two-way radio communications failure,
  air traffic control is based on anticipated pilot actions.

In other words, ATC expects IFR pilots to follow lost comms procedures as per 14 CFR 91.185 and will direct other traffic accordingly.
Finally, one 'routine' lost comms scenario is when ATC can't contact an aircraft because of poor radio coverage. This happens a lot with smaller aircraft (which fly relatively low) operating in remote areas and ATC might ask another aircraft, usually a higher-flying airliner if possible, to contact the first aircraft and relay their call: N12345, this is BigAir 123 with relay from Denver Center.

As you note, there are procedures to follow in lost comm scenarios, and ATC will expect you to follow these procedures.  In turn, they will continue to hand you off to subsequent controllers toward your destination and keep other aircraft out of your way.  They will attempt to establish one-way comms or comms via alternate methods (ACARS, SELCAL, satellite phone).  If comms are impossible, they will work other airplanes to maintain separation with you and the ability to do this easily comes from established lost comm procedures for pilots to follow.
For IFR lost comms, the procedures they expect you to follow are given in 14 CFR 91.195.
If you are in VMC or encounter VMC you will continue flight under VFR land as soon as practical.  If ATC sees you break off of your route and descend toward an airport, they will coordinate to make sure you successfully land and otherwise dispatch S&R to find you.
If you are in IMC or cannot comply with continuing under VFR (e.g. your operation is limited to IFR ops) then ATC will expect you to:

Follow the route:

that you were last cleared for
If you were on radar vectors, fly direct to the fix you were being vectored toward
If you do not have an assigned route, fly the expected route

and to:

Fly at the highest of the following altitudes:

Your assigned flight level or altitude
The minimum altitude for your operation
The expected flight level or altitude

The regs basically boil down to "fly your clearance and anything we told you to 'expect'".  If you lost comms shortly after takeoff while on radar vectors, you'd fly direct to the fix you were being vectored to (e.g. the first fix on your flight plan) while climbing to your expected flight level (e.g. your cruising altitude).  You'll then fly your cleared route to your destination, descending in accordance with your arrival procedure and then flying direct to an approach fix and flying the approach.
ATC will expect this from you and they will vector / sequence other airplanes out of your way to maintain separation.  They can reliably do this because your behavior is well-defined and as long as you do what is expected of you, you are relatively easy to work around.

the question is what will ATC do not what are pilot expected to do.

@ratchetfreak ATC will expect you to fly in accordance with the regs while they make sure no one is in your way.


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Are there runways that are numbered only on one side?
For all runways I've known or seen, they're numbered on both sides, with the number on the opposite direction differ by 18.
Are there runways that are numbered only on one side? I.e. due to certain constrains, the runway can only be used in one direction and never the other.

EDIT: I'm looking for runways where the aircraft is always pointing at the same direction. E.g. only runway 9 is available, you can never use runway 27 for whatever reason.
A runway that is blocked on one end will still have two numbers, since you'd land at one direction and takeoff the other.

if you can land one way you can take off in the other.

Does an aircraft carrier 'runway' count? :-)

@Pondlife I was thinking the same, but I don't think hull numbers count! :-)

@ratchetfreak You normally can, but that doesn't necessarily mean you're allowed to.

Well, here's one example, but I'm not sure if it counts, since the reason only one side is numbered is that the airport is closed and a large school was built where the other end of the runway used to be. :)

Looking at the answers, I think there's some confusion here, perhaps kevin can clarify. Are you asking about a runway - let's call it 18 - where both landings and takeoffs take place on 18, and no operations at all are allowed on 36? Or, are you asking about a runway where all operations take place on the same 'end', i.e. takeoffs on 18 only and landings on 36 only?

@reirab - that's really funny!

@Pondlife edited, I'm referring to the first case.

Yes, there are runways numbered on only one side.
For example, the third runway in the Frankfurt Airport (18, Runway west) is used only for takeoffs (to the south) and not for landings. Hence only number 18 is used. The other runways are used for both takeoffs and landings and are numbered 07C/25C, 07R/25L and 07L/25R.
Thomas Römer/OpenStreetMap data [CC BY-SA 2.0], via Wikimedia Commons
This restriction is due to the fact that it will interfere with the traffic in other runways.
Number 36 is not painted.

This is interesting and a little surprising. It seems like 36 would be numbered, even if landing on it were only available for emergency use. If a plane needs to land in an emergency, the other runways can be closed anyway, so the conflicting patterns wouldn't be an issue in that case. You are right, though. A Google Maps search shows 36 really isn't numbered.

@reirab Heathrow is similar.  09L is only used for landings due to noise.  Occasionally for ops reasons, take-offs are allowed from 09L and landings on 09R when the controller is in a good mood and the PIC asks nicely :)

@Simon It's not unusual for runways to be normally used for only take-off or only landing (that's pretty common at big, busy airports,) but it is unusual for the other end to not even be numbered. Both of Heathrow's runways are numbered on both sides.

@reirab Yes, I know.  I was just adding to the examples of where ops might not be from either end.

@reirab, alas it indeed is not. look at the aerial photograph of it.

Actually, you've got the wrong image, because it clearly does include the number 36 on the south end of the runway. However an aerial photography (see previous comment) proves that it is indeed not painted on the ground.

Can anyone explain the math behind why the two digits of opposing runways always add up to the same number? 36/18, 3+6=9, 1+8=9. 06/24, 0+6=6, 2+4=6. With the exception of 01/19

@TomMcW, there is no interesting math behind it as the exceptions 01/19, 10/28 and 11/29 indicate. It is just a coincidence following from the fact that the difference is always 18 and why that is is already explained here.

Just for the sake of completeness: while 18 is for takeoff only 07L/25R is for landing only (but in both directions).

This explains why numbers would be lacking even for helicopters with no approach concerns...

It is obvious from the map that there are too many problems with departing this runway as a hypothetical 36. But it's not obvious from this map why you couldn't have approaches and land on it. The only thing I see south of the runway is a nature preserve. Is there perhaps a law against flying over this forest?

@MichaelHampton: Think about it - where would you end up in the event of a  runway overrun?

@Sean You're quite right. Sometimes I do need to be smacked in the head with the obvious.

@MichaelHampton: Not that that always stops airport designers - Milan Linate, for instance, has (or at least had) a baggage-handling building off the departure end of runway 36R.  Why this is a bad idea was demonstrated, somewhat spectacularly, in 2001.

@Sean Now that you mention it, there's also TAM Flight 3054, which overran the runway, crossed the road at the end of the runway and ended up in a warehouse.

@Simon I wouldn’t say LHR 09L is similar to FRA 18. LHR 09L is regularly used as 27R and more importantly, is declared (though not always used) as takeoff and landing runway in both directions. FRA 18 is declared as unidirectional takeoff runway only (no landing whatsoever, and no 36 declared).

@aeroalias Regarding The other runways are used for both takeoffs and landings: That's wrong, 07L/25R is used (and IMHO certified) for landings only.
And regarding This restriction is due to the fact that it will interfere with the traffic in other runways: That's correct but for FRA there is also not only possibly interfering traffic but also building structures which would be too dangerous in case of runway overruns. The latter is IMHO the actual reason why 36 is not painted because even in emergency situation with all other traffic halted, this would be too dangerous

A mountain can get in the way for example at Lukla airport

The topography of the place makes any go-around impossible. At the
  south, the runway is the end of an angled drop of about 2000 feet.
  This cliff is fenced off as a precautionary measure. At the northern
  end of the runway there is a huge mountain terrain. It leaves no room
  for error.

source
Planes land on runway 6 and take off on runway 24.

 most information copied from this answer

This runway is still numbered on both sides, though.

Not only is the runway numbered on both sides, it is used in both direction—for take-offs in one and landings in the other.


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Are centrifugal compressors/turbines less efficient internally, and if so, why?
From my related question which found a good example of a centrifugal turbine jet engine, the Henkel Hes 3.

Picture Source Website (it's in Czech). Thanks to Jan Hudec for finding it.
My understanding of centrifugal engines is that they have larger diameters and therefore larger nacelle drag, and therefore aren't favored.
So in this question, I want to ask, are centrifugal engines less efficient internally? That is, for a given thrust at given conditions, will the centrifugal have more, less, or equal fuel consumption than a "normal" axial-flow engine?
I can't imagine that friction of air flowing around a winding path, or related viscuous effects, could amount to any noticeable penalty. But I can't think of any other reason why centrifugals would be internally less efficient.
Note, another related question, one answer did say that centrifugal compressors will have higher losses than axial compressors, for a given compression ratio, but did not explain why.

Internally the big difference is that with axial compressors, it's easy to have a multi-stage compressor to give considerably higher overall compression.
While it is possible to link centrifugal compressors together into a multi-stage engine, it's considerably less common, so most centrifugal compressor engines are limited to the compression they can get from one stage (usually around 4:1).
Axial flow engines get lower compression per stage (usually around 1.2:1), but it's easy to have multiple stages, so engines with 10-15 (or so) stages are fairly common. Given the difference in compression per stage, an axial flow engine typically needs about 8 stages to produce higher overall compression than a single-stage centrifugal compressor.
It is also possible to combine the two. At least to my knowledge, the most common layout in this case is multiple axial-flow stages, followed by a single final stage using a centrifugal compressor.
Here's one paper about using a final centrifugal compression stage, in case you care: https://doi.org/10.2514/6.2019-0944

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How do fuel tanks prevent free surface effect?
Free Surface Effect is a fancy term that refers to liquids changing their relative orientation within a container, whenever the container is rotated.
So imagine a large plane banking very steeply, at least 30 degrees. If the fuel tanks in the wing were one giant tank, all the fuel would run down to the end of one wing, and in the other wing the fuel would bunch up near the fuselage. This can change the center of mass and might be dangerous.
How is this prevented? Are their baffles in every fuel tank? Are there many individual tanks instead of one or two large ones? What sort of other designs might address this issue?

When you are in a 30 degree coordinated turn, do the passengers fall to the lower side of the aircraft? Yes there are baffles, yes there are multiple tanks...

It's not from banking that makes fuel slosh around when the airplane makes normal coordinated turns, any more than your drink sloshes over the side of your glass, or falls off the tray during a turn.  The fuel doesn't know there's a turn going on.
It's from yawing motions that occur, to the extent that the yaw damper system allows it (very small yaw excursions), or more severely, in the initial moments following an engine failure where big yawing motions occur if the pilot is slow in correcting the asymmetric thrust, and perhaps during landing where yawing actions are done to align with the runway.
In any case, the fuel tanks are just sealed off areas of the wing box structure, with bulkheads formed by the ribs.  The bulkheads will have holes to allow fuel to migrate, so they act like baffles.  In some bulkheads, there may be little one-way flapper valves along the bottom to only allow the fuel to migrate inboard, so once an outer zone of the tank is empty, fuel can't slosh back into it.
At the inboard end of the typical wing tank is a "collector tank" that the engines actually draw from, which is continuously filled to overflowing (the overflow running back into the main tank or sometimes just overflowing an opening around the top within the main tank).  The filling of the collector tank is done by scavenge pumps installed at the tank extremities.


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Are tripropellant jet engines a viable way to increase fuel efficiency for large commercial airplanes?
I was just reading about a tripropellent rocket engine that apparently passed all tests and was ready for flight before the program was cancelled in the early 90's. The engine was the RD-701 for the MAKS spaceplane program. It used kerosene and liquid oxygen before switching to liquid hydrogen.
So I'm now thinking about tripropellant jet engines. If we had a small tank of liquid hydrogen to either mix with the regular jet fuel or replace it for a short while, would this viably improve fuel economy for large commercial airplanes?
(I've heard that jet engines can easily run on almost anything that burns, so presumably it would be easy to get a jet running on hydrogen. Correct me if I'm wrong.)
There are many types of efficiencies, so let me be clear about which one I'm interested in: Fuel economy, as in miles per passenger-gallon.
Let's ignore any logistics on the ground about obtaining and storing the hydrogen. However, storage aboard the airplane itself should certainly be considered and might be a very detrimental factor, as might flammability. I'm pretty sure we would at least need double-walled, vacuum insulated tanks to let us store that hydrogen throughout the flight. They might be twice as heavy as normal tanks.
If you know anything about hydrogen, its chemical energy (per mass) is greater than gasoline....by a factor of 3 IIRC. The Lower Heating Values are around 120 versus 40. So we might all ask why not just use liquid hydrogen in everything and what is taking so long? Well apart from cryogenic considerations, it's not very dense at all. We would need a huge tank maybe even bigger than the airplane itself to store the necessary hydrogen, even tho that huge volume of hydrogen would still weigh less than the necessary jet fuel.
I think the idea of tripropellant engines is to allow some hydrogen with a not-so-overwhelming tank. The tank would only be as big as we can reasonably handle. Therefore we use some hydrogen to get a bonus in fuel economy.
P.S. I'm not really asking this from a "green" perspective. Hydrogen and oxygen burn to form pure water exhaust, which sounds so harmless, but water vapor is actually a very potent greenhouse gas, much more potent than CO2 molecule-for-molecule---especially when released at high altitudes. I'm asking this purely from a fuel-economy standpoint.

Don't forget the added complexity factor. Never a good thing on aircraft

Related: Cryogenic Aircraft on Global Security. It is anticipated that by 2040 the use of kerosene will be difficult (resource exhaustion and greenhouse effect), LH2 could be used if confirmed.

Hydrogen indeed has 3x the energy density per unit weight, but alas, 1/3 the energy density per volume.  Cost and weight (of fuel tanks and insulation) are issues here, but LNG is very inexpensive (and not nearly as cold).

First of all: jets are not "n-propellant". Rockets are bipropellant. That's because rocket needs to carry fuel and oxidizer, while air-breathing jet gets the oxidizer (plus huge amounts of reaction mass) for free from the air. So jets carry only one propellant (which doesn't make them monopropellant either). The tripropellant rockets were imagined to overcome problems with specific impulse. The most complicated and impractical rocket engine could go as high as 542s - while average jet operates around 3000s easily. So specific impulse is the problem jets never had in the first place.
The biggest advantage of RD-701 was to use the kerosene-hydrogen mix on low altitude (where kerosene is more efficient) and then switch to pure hydrogen in space (where hydrogen is more efficient). They started with a hydrogen engine (RD-0120) and then added kerosene to increase efficiency by reducing the amount of hydrogen needed - so the exactly opposite reasoning. The main problem with your idea here is that jets never leave the kerosene zone.
The main problem of liquid hydrogen is that the fuel is lighter, but the tanks are so heavy it completely obliterates any savings. Rockets here get it easy - rocket working lifetime is measured in minutes (Space Shuttle burn: 8m30s Soyuz burn: bit over 10m50s). A plane on the other hand would need to store the hydrogen for a dozen of hours = even more weight. There is no weight-efficient way of keeping the cryogenic fluid on the go. All you can do is to let some of it "boil-off". Twice as heavy as kerosene tank is a huge understatement IMHO, as in modern jet liners the tanks - "wet wing" - are almost free (in terms of mass). Can't do that with hydrogen - you'll need to sacrifice cargo space for dedicated LH2 tank.
Another issue is that hydrogen burns in higher temperature. We're already having problems with making the turbine survive kerosene temperatures, adding hydrogen would make the engine design even more difficult. From the green perspective it's not all roses either, because the higher combustion temperature, the more NOx created.
I have to agree with others here - hydrogen planes won't happen unless we'll run out of easier options. Having 2 kinds of fuel is the most difficult of them all.

On the other hand, store enough hydrogen at low enough pressure, and the additional weight is negative...

@BrianDrummond So is the cargo/passenger space : )

@BrianDrummond Yes, and that works great if you plan to stay at ~70kt top speed and a few hundred feet AGL. Well, until it explodes, that is.

From the green perspective it's not good either, because the higher combustion temperature, the more NOx created. That may be a severe simplification. Burning hydrogen produces no CO2 and no carbon monoxide either, and they have greater effects at higher altitudes. It would take a dedicated quetion on this site, along with a complex answer, to see how much extra NOx is produced and if it outweights the reduced carbon effects, or not.

Yes, you can reduce takeoff mass by exchanging kerosene with hydrogen, but only above a minimum volume where the increased weight of the tank does not cancel out this advantage. But any manufacturer introducing such an aircraft today would immediately hit the problem of fuel availability - there is simply no infrastructure for hydrogen at airports.
Historically, the military paved the way for infrastructure improvements, and with aerial refueling, satellite technology and the vanishing importance of long-range platforms (except for unmanned reconnaissance maybe) there is no pressing need today to roll out the hydrogen infrastructure.
Note that project Suntan (which became the Lockheed CL400) invested into the liquid hydrogen infrastructure that enabled the use of cryogenic rockets in the Apollo program. Maybe a future military hypersonic vehicle will do the same for hydrogen in civilian air transport, but I am skeptical that this will happen in my lifetime.

Can you put the "vanishing importance of long-range platforms (except for unmanned reconnaissance maybe)" statement in context? That seems somewhat counterintuitive. Are you referring to a trend that's disaggregated from the effects of aerial refueling technology?

@Sorghum Most of what satellites do today was done by long-range platforms in the past. They simply have become obsolete, except for dynamic situations where a surprise overflight might reveal things that an easily predictable satellite would have missed. Cruise Missiles do what used to be done by long-range bombers and civilian air transport was excluded from that statement as it refers to the development of infrastructure by the military.

I figured your statement specifically referred go military aircraft, so no worries. Thank you for clarifying.


text:
Why do many large turbofans have a gap (of turbine blades) between the LP turbine and HP turbine?
After browsing around a while, I noticed that many...I think almost all...large turbofans have this gap between the LP turbine and HP turbine. I took some screenshots and put a red circle around the gap.

Why do they have this gap? It's certainly not for the combustion chamber. That part comes before the LP turbine. The only thing I can think of is the LP and HP are on different spools so maybe they need some space for the bearings to brace the two shafts. But there should be bearings bracing the shaft all along its length so idk.
I'm guessing it would be more efficient to eliminate this gap and should allow a smooth transition from high pressure to low pressure. At the very least it would save some room at the end of the engine, shortening its length and maybe saving some weight.
EDIT: At first I thought it was totally empty. But it appears that space has stator vanes not shown in the pictures. I'm editing the title with the phrase "gap of turbine blades" because the original spirit of the question was about way there's a big space between the two sets of turbine blades. The space happens to have something else in it, but not turbine blades and I was wondering why those sets are separated.

Because the fan runs at a much lower RPM than the compressors, so for a un-geared LP turbine to fan link the LP turbine needs to run at the same lower RPM. As a result the HP and LP turbine has a much difference in diameter so you need some space for the diameter transition. Filling the gap with either LP or HP blades doesn't worth the weight or complexity. Some turbofans has geared LP turbine to fan connection so the LP turbine runs at a much higher RPM hence a smaller diameter, then you won't see the gap on them.

@ymb1 is correct that there is an aerodynamic function of the LPT nozzle in that space, but there is also a mechanical function as well. You correctly observed that there will be bearings supporting the shaft. But what supports the bearings? They are not just floating out there in space.  The bearings are supported by a frame.
On the GP7200 and CFM LEAP, there is a frame between the HPT and LPT that is referred to as the "turbine center frame".  The function of the frame is to provide a stiff loadpath from the bearing that supports the aft end of the HP shaft up to the main structure of the engine that connects to the aircraft. If you check this post, you'll see plenty of examples for the frames.

Short answer
This space is part of the inter-turbine duct (ITD), a duct which is required to maintain a satisfactory flow continuity between HPT and LPT rotating at different speeds (in the ratio of 1:3) and having different diameters. The duct is made of the gap and the larger (in the axis direction) LPT stage 1 vanes.
Note this duct is a diffuser as the annular area is growing along the flow path. What happens in this diffuser is not very well known and its design has been empirical to some extend, using guides like this:

Pressure recovery vs friction. Source
Some numerical simulation demonstrated the design indeed influences the LPT (pressure loss in the duct), but also the HPT and the combustion. On days where saving 1% fuel (therefore 1% CO2) is important, some effort is now done on the optimization of the inter-turbine duct.
Shortened ITD are complex to design and known under the name of aggressive ITD.
Details
Taking the CFM56-7B (Boeing 737 NG) as an example. This engine has a single stage HPT and a 4 stage LPT.
Each rotor is preceded by stator (nozzle). The role of the stator vanes is to convert pressure into velocity and turn the flow more perpendicular to the rotor blades surface. This is opposite to the compressor stages where the vanes are downstream of the blades to convert velocity added by the blades into pressure, maintain velocity constant and straighten the flow.
Gap and larger vanes
Everything is globally continuous, though there is a a gap forward of the LPT stage 1 nozzle located in the HPT casing. The vanes themselves are larger axially.

CFM56-7B, small gap before LPT Stage 1 vanes
This location is called the inter-turbine (transition) duct, it corresponds to the interface between HPT and LPT which are rotating at different speeds.
The duct has an increasing area due to the larger diameter and size of the LPT stage 1 rotor (a similar duct exists between LPC and HPC for the same reasons).

Inter-turbine duct geometry. Source
The gap is chosen to set (empirically) the adequate duct slope, aspect ratio, etc for an acceptable pressure loss between the turbines.
To increase engine performance, manufacturers are trying to decrease the length of the duct but there is a lack of knowledge in this domain.
Shorter ducts are known as aggressive inter-turbine ducts (AITD). See this paper.
Bearings
Regarding the reason mentioned in another answer, a possible problem with a bearing or a bearing support in the way: This is not actual, the bearings are much smaller than the turbine stages:

CFM56-7B, location of rear bearings

The No 4 bearing is between the HPT shaft and the LPT shaft.
The No 5 outer support is within the oil collector, and the bearing itself is within the its support.


text:
How are naval jets moved around below the deck of a carrier?
There are many kinds of jets on an aircraft carrier. I'm curious how they're moved around below deck. They have to be moved around for storage and to get to and from the large elevator that takes them up to and down from the flight deck.
So how are they moved? Is there some tug? Or something powering their wheels? I really hope they don't have to use their jet engines below deck, as that seems very dangerous to me.

Using the jet engines would be not only dangerous, but also extremely fuel-demanding (sure more than even a fossil power plant; not to speak a nuclear power plant). There must be something else into it...

@yo' It would absolutely be fuel-demanding and I would be shocked to find out if they actually would use jet engines below deck. But the only other things I have are guesses, just my 2 cents.

They use tugs of all sorts. Here's another example

@Simon That's pretty cool but how fast and efficient are those tugs? What is their power source? PS, this could be an answer.

@DrZ214 Well, I only spent a couple of days on HMS Illustrious and saw tugs being used (and occasionally man power) but I don't have enough information to make a proper answer - other than "they use tugs" :)  As for power, from memory, the small ones where battery powered and the larger ones, diesel.

As Simon pointed out, they use tugs. Obviously the engines are not started below the deck as it would have potentially deadly consequences.
The tugs are either electric or powered by a diesel/JP-5 engine. For example, the USN uses the A/S32A-32 towing tractor (also called spotting dolly SD-2), among others.

Image from zone-five.net
This tug is powered by a diesel engine and controlled using a joystick. In order to be useful in the confines of the deck, the tug can rotate with a zero turning radius. The operation is pretty much similar to the other tugs- the tug is moved so that it lifts up the nosewheel, facilitating the aircraft movement.
The French, on the other hand use remotely controlled mototk tugs on board de Gaulle.

Image from mototok.com

This is what I'm looking for, but questions remain. How does a diesel-powered tug running below deck not asphyxiate or carbon-monoxide poison everyone below deck? Do they all wear gas masks? Also, that smaller one in the last photo...how does such a small thing get enough traction to pull around a fully loaded fighter jet?

The hangar bay of an aircraft carrier has four giant doors which are (almost) always open. Plenty of fresh air.

On the old carriers that had a center deck elevator planes would spin on the deck and get into position to taxi into hangar bay one! You wouldn't believe how loud a "screaming Demon" could be! I was on the Oriskany in 1963 when they were still doing but later that year I was on the Midway and they stopped doing it!


text:
Can we eliminate the center tank of large passenger planes?
This is a safety question about fuel and fires. This all regards commercial aviation for large passenger planes.
Most fuel is stored in the wings, but in the largest planes, there's also a center tank for the longest flights. It's this center tank I'm concerned with. In the event of a fire, this seems to be the most dangerous tank.
I'm obviously concerned with fires in the fuselage, or very close to it. Obviously this is where all the people are and we don't want smoke or flame there, but remember also there are overhead oxygen canisters that if heated or fueled, will explode.
Jet fuel is flammable by its nature. This cannot be changed because its purpose is to combust in an internal combustion engine. So it seems the best we can do is to keep the fuel as far from the fuselage as possible, to make it unlikely that fuel will cause a fire there (initially).
Isn't there someplace we can move this center tank? Some ideas I thought of: store fuel in anti-shock bodies on the wings (or they could be just small sears-haack bodies on the wings); store fuel in the empannage; store fuel in nacelles that have a larger diameter than the engine itself, to make room for fuel.
Another exotic idea: Is it possible to engineer the wing such that the most likely structural failure will happen first near the wingtip rather than the wingroot? This would spill the fuel farther from the fuselage. (I got this idea from a really wierd Boeing 747 crash in Netherlands where one engine jumped off then fell back into the other engine, causing 2 engine losses. They said some pins were designed to fail first for a more "benign" failure, but apparently those pylon pins were engineered wrong.)
There's one more tricky caveat, though: the cross feed line to balance fuel loads in the wings. If we eliminate this line, we increase the dangerous consequences of a fuel imbalance. But, it's interesting to think of what might cause a major fuel imbalance other than a leak, in which case the cross feed valve should not be opened. I can't think of anything so I'm leaning toward the opinion that this line can be eliminated too.
EDIT: Thanks to bodargpd for pointing this out. Losing an engine in flight means the consumption rates will be hugely different. Without a cross-feed line, all that fuel in the other wing is unavailable and we may have to ditch early. So the cross-feed line is still necessary. I still say this is much less dangerous than a center fuel tank, however. It would also be nice if we engineered the cross-feed line to be weakest at the sides, so any leak will most likely be as far to the outside of the fuselage as possible.
To sum up, the basic question is, can we viably move the center tank volume of fuel somewhere outside the fuselage?
EDIT: I'm not primarily worried about fuel explosions. I'm concerned mostly about fuel leaks during hard emergency landings. I'd rather have a fuel leak out on the wing than in the fuselage.

If it explodes in the wing or the fuselage the effect will probably be the same,  and considering how rare center tank explosions are (I know of two examples,  only one,  TWA 800, was fatal), not sure this is really a major issue. The bigger issue is empty tanks, not full ones.

Let's say you lose the left engine on a two-engine aircraft. You'll drift down to a lower altitude and be at a lower speed. If you eliminate the cross feed line, you have lost access to all the fuel in the left wing, and being now low and slow, you may need that fuel. If you're in the middle of an ocean, being unable to use that fuel may mean you'll have to ditch. Just as a matter of an old man's opinion, I spent 10 years on 747-100/200 aircraft with center tanks holding, as I remember, around 340,000 lbs, and I never felt uneasy about that, even after TWA 800.

I believe most commercial pax flights use chemical oxygen generators and do not have compressed O2 stored in bottles above the seats. I would imagine that those are far less dangerous in a fire than the O2 cylinders.

@FreeMan Not necessarily; chemical oxygen generators are still providing oxygen. Chemical oxygen generators feeding a cargo hold fire were the problem in the ValuJet Flight 592 crash, for example.

@bogardpd It was a problem because they were incorrectly packed, carrying too many, and were stored next to a tire. When they are installed they generate the same amount of heat, but they are not a fire hazard.

How do you propose to engineer a wing to fail at the tip rather than the root? There's very little load at the tip so there's nothing to make it fail there. Indeed, whenever something strikes the wing, the wing almost has to break somewhere between the impact point and the root, because that's where the torque is being applied.

Why can't planes use F1-style kevlar tanks for fuel, which would stop them from being incinerated in case of a crash

We can solve most cases by deciding to burn the center tank first. For the one engine out on takeoff there's always dump fuel overboard.

@RonBeyer: Not necessarily - wing tank explosions are, occasionally, survivable.

@RonBeyer I remember being told that planes don't "explode", they just have an intense fire.

I'm assuming you're talking passenger safety in the event of a ground impact - if anything happened midair, it wouldn't really matter if it were in the fuselage or one of the wings.
Moving the fuel somewhere else is going to have center of gravity implications. Moving it to wing pods or lager nacelles in particular would increase drag, which would require the plane to have even more fuel on board, which seems counterproductive to decreasing the fuel risk to passengers (and, of course, would substantially increase the cost of operating the flight).
Some military aircraft do carry fuel in removable underwing fuel pods (particularly fighters being ferried somewhere, where increased endurance/time between refuelings is more important than performance for that particular mission). However, that's less a safety design and more the lack of anywhere else to put the extra fuel on a small aircraft.
If an engine fails, the rate of fuel consumption is going to be different on each side of the plane. Although both are rare, you likely have far more situations where different wings use fuel at different rates than situations where having fuel feed under the body would be dangerous. If that's true, then removing the cross-feed line would be more dangerous than leaving it in.

Yeah I'm worried about ground impact (hard landings in emergencies), not fuel explosions mid-air. I edited the OP.

Removing the centre fuel tank doesn't mean you have to have no feed line between the two wing tanks. If we accept the hypothesis of the question, it's surely safer to have a fuel pipe passing through the fuselage than to have a tank containing thousands of litres of fuel there. That pipe would be protected by closed valves at each wing tank so would have a relatively small amount of fuel in it, compared to a tank.

@DavidRicherby: You mean like the tank arrangement on the 777-200?

Extending bogardpd's answer: moving the fuel from the center to the wings will also upset the weight and balance. When the plane rolls, since the mass is further away from the rotation center, the moment of inertia is increased. Which means slower response and a need for stronger wings, which means more weight, which means even stronger wings...... You get the idea.

But weight in the wingtips enhances roll stability-- or so we can read on Aviation Stack Exchange

having more of the fuel on the wing gives bending relief, reducing weight........?

Yes the fuel from the centre tank can be moved to another place in the aircraft. It's a lot of fuel though.

Some ideas I thought of:

Store fuel in anti-shock bodies on the wings (or they could be just small sears-haack bodies on the wings). Technically possible, but there will be a large amount of pods.
Store fuel in the empannage. This is already done for cruise trim drag elimination.
Store fuel in nacelles that have a larger diameter than the engine itself, to make room for fuel. These would be huge and the fuel would be directly around the heat source of the combustion chamber.

All of this is technically possible, but is there really a problem to solve? The centre tank is the first one to be emptied, since the fuel in the wing provides bending relief.
All solutions would introduce economical issues: the extra drag makes our tickets more expensive.

Is it possible to engineer the wing such that the most likely structural failure will happen first near the wingtip rather than the wingroot?

The root of the wing has the largest stresses due to bending moment, and will unfortunately fail first.

Probably safer in the belly than anywhere else due to stress, bending, and weight balance issues.  The wings would be closest to pitch CG, but loading them with fuel would affect the roll rate as well as a possibility of creating a sickening uneven "sloshing" motion that could affect passenger comfort.
The solution is called "inerting" or eliminating one of the spark, oxygen, fuel components that
a fire needs.  Nitrogen will not support combustion of jet fuel, so filling the fuel tank with nitrogen greatly improves safety.  Nitrogen is cheap, and a gentle flow into the tank will keep even volatile liquids from igniting.

Bending is not a good argument since bending relief is the reason wing tanks are emptied last. The underside is also the most exposed area of the airframe in the event of a crash or even a gear-up landing, so I don't buy the "safer" argument. Regarding inerting, nitrogen is cheap, but the tanks are not, weight-wise, see this report: https://www.fire.tc.faa.gov/pdf/TG3.pdf

@AEhere I understand spanwise loading provides bending relief for wings, but what about fuel lines?  Wings do bend.  Maybe I'm old fashioned, but putting the fuel tank where the bomb bay was seems good!  I would avoid crashes too, but one Russian CNG design did put the tank ABOVE the passenger compartment.  Final note about "sloshing" or "Dutch Roll", getting that weight in the center and building good strong wings would be my approach.

@RobertDiGiovanni uh that increases weight. And there's a bigger swimming pool for you to fall in in a crash.

Inerting (after purge) requires very little flow.  I used to measure nitrogen flow into my reactors just by watching it bubble through an oil trap.  Weight issues would be minimal.


text:
What is the unit for the oil quantity display in the Airbus A320 ECAM?

Is this image showing 18.5 liters or quart of engine oil? It is from an Airbus A320 with CFM 56-5B engines.

The answer is actually in the image: Below "OIL", it says "QT", which according to the "Abbreviations" table under "General Information" in the A320 FCOM, means

Quart (US)

Thanks You. You are great

Probably they use this unit because as it is close to liter (0.94), they don't have to change anything for the european market, except the label.

@Legisey I think the European market also uses quarts, not liters. It wouldn't make sense for a European made plane which is used by airlines around the globe and potentially flies intercontinental with pilots from all over the globe to have separate displays for different markets, especially in the risky manner you describe where they only replace the unit but don't redo the calculations.

A bigger question is why do they use US Quarts for oil? Or more broadly how have we ended up with such a mix of US and metric units (and possibly imperial for all I know).  I was going to ask but it's too close to Why doesn't the aviation industry use SI units?, thoughg the accepted answer there states Non-SI is only used for altitude, distance and speed except in US and some other American countries. which doesn't seem to be the case here

Yes. The capacity of tank for overflow oil is 20.1 quart. And why i uplift full oil, it only show 18.5. There are logical about it?

The ECAM engine oil quantity indication corresponds to the "useable" oil quantity in the
engine oil system
The difference between "total" and "useable" oil quantities is around 5 US quarts.


text:
Which American twin tilt-rotor aircraft is this a scale-model of?
What aircraft is this model?

(photos: own work)

The registration decal 227316 seems to have been pinched from a P-47 model with a rather excellent native American paint scheme
https://www.scalemates.com/kits/academy-2174-p-47d-thunderbolt-bubble-top--107421

Looks like the Bell XV-3, "an American tiltrotor aircraft developed by Bell Helicopter for a joint research program between the United States Air Force and the United States Army in order to explore convertiplane technologies."

Bell XV-3. The Transcendental Aircraft 1-G was slightly diferent. Kind of H-34 Choctaw looking compared to the XV-3.

Welcome to Av.se. Can you elaborate any on your answer, maybe explaining where the two designs differed & what features in the OP's photo are more similar to the XV-3? As it stands, a reader has little basis to see the validity of your answer, beyond having "read it on the internet".


text:
Does ducting help to stabilise heavy lift drones?
I'm looking to build a UAV that is extremely stable. It doesn't have to be fast, it doesn't have to be agile, but it will have to carry up to 50lbs (agriculture focused UAV that has to operate for at least 8 hours at a time) over private land (where the drone is authorized to be, and with FAA registration) at an altitude of about 15 feet.
Is ducting the way to go here?
I'm looking to do a quadcopter with a very very low center of gravity right now because of it's inherent stability. Is ducting or more rotors the way to go here? And I have very few specifics because I'm still working on the basics of the platform. It will have to carry a lot of weight and potentially deal with being thrown off balance by a rover getting caught on plants.

Just wondering, have you done any calculations to figure out how many watts you will need to keep the vehicle airborne?

No, I haven't. I'm guessing batteries are actually going to be most of the weight of it. I'm fairly new to this, perhaps unsurpisingly, so I may find that as I progress on this changes will have to be made because i'll find that my original goals are unrealistic.

As @qqjkztd said, that doesn't necessarily help stability. See the Pendulum Rocket Fallacy. Note that the idea applies to more than just rockets.

There is too little information given to make a definite answer here, but things you should take into consideration are:

depending on the construction the bigger crosssection if keeping a similar rotorsize which will likely increase drag at higher windspeeds while on the other hand leading the airflow so with the right construction it might behave better then without ducting.
using at least 6 or better 8 rotors should enable you to increase lifting power and stability
putting the rotors in an angled position should increase stability while it might slightly reduce lifting power

additionally

the ducting will offer protection for the rotors/fans and people/things getting hit by the drone in case of a crash
as you want too operate it for such a long time the increase in weight which might increase the number of refuels/battery changes needed

Also we were talking about drones that produce the whole lift by rotors pushing air downwards. What about using a ballon or some kind of plane? This will make it more sensitive towards wind and sacrifice maneuverability but will allow extended flight time and potentially higher efficiency.

Thank you for the information, sensitivity to wind will be a major issue. The purpose is for mapping moisture, ph, spotting disease and animal damage automatically and a few other things as well. It's going to have to hover in places while it drops a "rover" from it to take soil readings then retract it back up.

This sounds like a pretty advanced project to me as dropping and lifting a rover will cause forces that most drones are not programmed for. I guess the rover is done already? Also most drones have flight times under 30 minutes, landing or hovering only a few centimetres over the ground might be necessary if the rover takes a while to collect samples as otherwise you would have to carry insanely large batterypacks or use fuel.

The rover isn't done yet, I have some of the sensors and some of the software done. I can't think of another way to allow both normal farm equipment to operate, and to get the necessary readings than using. An aerial vehicle to navigate the fields themselves. I chose 50lbs as a somewhat arbitrary worst case scenario. I'll obviously do everything I can to keep it light. The UAV will stop drop the rover. And hover at around 15 feet. The rover needs to be able to drop to the soil extend it's sensors move off a lump of dirt that's in the way and go back up.

I've thought about using fuel but that seems like another layer of complication. I also thought about using mini jet engines for their thrust and control, but there's too much risk of fire in a dry field. So I think insane amounts of battery packs it might be. This would have to cover hundreds of acres at a time. I might be able to make it modular. But ease of use is key here. This is why I'm here though, I have plenty of experience in electronics, coding, and data modeling. Aviation not so much :-)

Does it have to be a rover or could it also just be a plate with like nails to provide grip and a drill? Either dropped with a wind by the drone or fixed to the drone which then lands on it. What kind of land are you measuring? Is it just gras or big stuff like corn?

Big stuff like corn is one of the major places to use this. And as of 2014 the average size of a farm in Kansas was 747 acres. Eventually it would be linked with a database and could recommend different varieties of corn or even new crops based on the soil conditions. And it doesn't need a drill. It needs to be able to move off of lumps of dirt though so it doesn't get stuck.

Also this sound like a rather expensive drone, you might want to ask an engineer to run a simulation, after a physicist checked your number are right

Might need to. So far I'm only in this project about 100USD. Goal is to keep it under about 3k total.

Ducted fans, as far as I know, don't increase the stability of a drone. They do have a positive effect on the efficiency of the propellors, if and only if, the airflow is more or less without any incidence (where the "more or less" is heavily effected by the shape of the ducts). So for outside use, as well as racing etc. ducts are not the way to go from a efficiency and/or stability perspective. Safety considerations are still a valid reason to choose for them.
Regarding the low center of gravity, make sure you don't overdo it. While it does increase the stability, it also makes the system slower to compensate, which will be felt in gusty conditions. It will be worse at keeping position.


text:
What are the equations of motions for parafoils?
I want to use the following equations so as to describe the motion of parafoil
$$\left[ {\begin{array}{*{20}{c}}{\dot x}\\{\dot y}\\{\dot z}\end{array}} \right] = {V_a}\left[ {\begin{array}{*{20}{c}}{\cos ({\gamma _a})cos({\chi _a})}\\{\cos ({\gamma _a})sin({\chi _a})}\\{ - sin({\gamma _a})}\end{array}} \right] + \left[ {\begin{array}{*{20}{c}}{{w_x}}\\{{w_y}}\\{{w_z}}\end{array}} \right],
$$
where ${\gamma _a}$ is a flight path angle, and ${\chi _a}$ is heading angle in horizontal plane.
$${\gamma _a} =  - arcsin \frac{{{V_{az}}}}{{{V_a}}}$$
and also
$${\gamma _a} =   arctan \frac{{{-1}}}{{{L/D}}}$$
The equations for lift force and drag force lool like these
$${\displaystyle L\,=\,{\tfrac {1}{2}}\,\rho \,V_{a}^{2}\,C_{L}\,A}$$
$${\displaystyle D\,=\,{\tfrac {1}{2}}\,\rho \,V_{a}^{2}\,C_{D}\,A}$$
according to paper https://lib.dr.iastate.edu/cgi/viewcontent.cgi?referer=https://www.google.ru/&httpsredir=1&article=1629&context=etd
But why the reference area $A$ and speed is similar for both cases? I think its projection should be rather smaller for drag force than for lift one.
But then on can make assumption that
$${\gamma _a} =   arctan \frac{{{-1}}}{{{C_{L}/C_{D}}}}$$
I don't understand why this is valid for parafoil
Anyway, I don't want to sink in rigid body physics and it will be comfortable for me to stay on next level of abstraction.
For my case the maximun lift-to-drag ration
$L/D = 4$ and the only information i know is the area of parafoil.
It's enough to characterize the simple model but i can't describe the change of speed caused by the change of lift-to-drag ratio due-to symmetric motion of control handles (breakes, srtopes? I'm not sure what defenition is correct). What is the equation that is responsible for the dependence of lift-to-drad ratio on symmetric motion of handles? Or is there other way to write down variation of velocty due-to handles lengths that allows not to consider forces? It would be appropriate if you know how it was done in X-38 parafoil.
In my opinion the shoud be the law like this
$${\gamma _a} = {\mathop{\rm artan}\nolimits} \left( {\frac{{ - 1}}{{L/D}}} \right) = f(\delta )$$ where $\delta$ is symmetric lenghth of each of two  control handles. But as I said I have only area of parafoil that is 400 ${m^2}$ and mass a few ton.

I don't want to sink in rigid body physics and it will be comfortable for me to stay on next level of abstraction.

Then I'm afraid you will stay stuck at your current level.
Your first equation is only a coordinate transform for the airspeed vector.
The reference area is just an arbitrary area. It makes sense to use the same area for all force components, and the wing area has been universally agreed for this. You will note that c$_D$ is quite a bit smaller than c$_L$ - why should the area be different?
When the same area and speed is used, you can calculate with coefficients just as if they were the real forces. This makes comparison between different speeds and different aircraft (or parafoils) much easier.
Regarding the effect of pulling on the risers (yes, that's the proper word): This changes both the load distribution and the local shape of the parafoil and is not easily expressed in a simple equation.

Peter, I'm glad you answered this, because as a knuckle-dragging left-seat-control-column-actuator, I have no real idea if this question is or is not (a) on-topic, and/or (b) too broad.  That many equations makes my head spin, and I've even had a little bit of Aero (a few decades ago). I suspect the question will end up being closed, but at least the OP has a good answer from one of the very few people here who could provide him one!


text:
Does FAA part 91 require aircraft flight logs?
What are the record-keeping requirements for flight or operational use of an aircraft only flown under Part 91? Rental aircraft always have such a logbook, for obvious reasons, but are privately-owned-and-operated aircraft required to keep such a log by regulation?
91.417 and 43.9 clearly dictate the requirements we often refer to as the airframe, engine, and prop maintenance logs. This is not the same as an aircraft flight log.
AC 43-9C further clarifies the above.
61.51 dictates pilot experience logging requirements. Some of the information recorded in a pilot log is duplicated in the aircraft flight log.
In Canada, this is referred to as a "journey log" and the requirements are explicitly stated in 605.94
AC 91-70B concerns flying in "oceanic and remote continental airspace", but deep in 5.4.1.2 recommends a journey log and references ICAO Annex 6.
ICAO Annex 6 Part I 11.4 defines the "journey log" for commercial flights, and is a "recommendation".
ICAO Annex 6 Part II 2.8.2 similarly defines the "journey log" for general aviation, and is also a "recommendation".

I cannot find an open access reference to link to the ICAO Convention on International Civil Aviation. It appears to be a document that can be purchased for a fee.


text:
Use of a METAR when $ is present
I fly out of an uncontrolled airport that has an unmanned, automated weather reporting site with a precipitation sensor (AO2).  The METAR often has a \$ symbol at the end of the report, indicating that maintenance is needed on the equipment.  Can I use this METAR, or any METAR with a "\$" as a reliable indication of the current weather?

Related: What does a dollar sign mean in a METAR?

Do the indications match up and make sense compared with indications of a nearby airfield and other weather sources?
Legally the FARs require that pilots obtain "all available information" (FAR 91.103). $ could mean a number of things and since Metars are still available, you should be able to trust them, but I recommend to check more then one source regardless.


text:
How come certain airports can have unique rotating beacon colors?
The Hershey Medical Center Heliport has a white-red-green beacon as opposed to a normal heliport white-yellow-green beacon.
How is this allowed?

Good question. A search of AirNav.com shows that it is the only one in the US with  those colors.

Maybe they're decorating for Christmas? :P

The lighting for private heliports is not regulated by the FAA, but by the individual states. The FAA has an Advisory Circular laying out guidelines for heliports, but it is only a recommendation.

The  Federal  Aviation  Administration  (FAA)  recommends
  the  specifications
  contained in this AC for all applica
  tions involving development of this natu
  re.  In general, use of this AC
  is not mandatory.
From FAA AC 150/5345-12F: Specification for Airport and Heliport Beacons

It is only regulatory when the heliport is built using federal funds. In the AC they recommend the white/green/yellow pattern.
Only a couple of states have adopted the FAA guidelines as regulatory.
The state of Pennsylvania incorporates many of the FAA Advisory Circulars in their state law. For example the ground markings and the lighting for the perimeter of the landing and takeoff area for the heliport are required to adhere to the appropriate FAA AC's. But in regards to the beacon it simply states:

A rotating beacon is recommended to be installed for night operations at public heliports.
Pa Code § 471, Appendix A, Criteria I, i)

It does not even have that recommendation listed for private heliports.
Although it is not in the FAA Advisory Circular some places list the beacon color code for hospital heliports as white/green/red. I can't find anything authoritative on this but examples are at an aviation wiki, on Quora and on airliners.net.
This company that sells heliport beacons indicates that their beacon is "Available in Hospital Heliport version (not FAA certifiable)." It doesn't indicate, but this is likely a white/green/red version. And since it doesn't adhere to the Advisory Circular it isn't FAA certifiable.

The AIM 13. Airport/Heliport Beacons has the colors for different types.          13.2.5 Green, Yellow, and White−Lighted heliport.

@JScarry AIM 13?

AIM = Aeronautical Information Manual https://www.faa.gov/air_traffic/publications/media/AIM.pdf

@JScarry It's not the AIM part I'm confused about. It's the chapter 13 part.. There are only 10 chapters

@JScarry Aside from the fact that the AIM is also not regulatory

My mistake AERONAUTICAL INFORMATION PUBLICATION, not the AIM

@JScarry Haven't seen that document before. It does specify in AD 1.1 sec 5.2 that for privately owned aerodromes that compliance is suggested and "Further compliance is by voluntary action on the part of the aerodrome owner."

So, basically, it sounds like most, if not all federal jurisdiction over aerodrome regulations is tied to the money. A very common way for the US Govt to get around the elastic clause in the constitution. If you don't take federal money you don't have to comply with federal regulations.


text:
How to call a cartesian (END) coordinate system, that is referenced in surrounding air?
I'm looking for the correct/usual terminology for a coordinate system, that is moving with the wind.
I do calculations in a Cartesian East-North-Down (END) system. When taking wind into account I'd like to move this system with the wind and transform it back to an earth-fixed END-system afterwards. So basically, the reference point of my coordinate system isn't a point fixed on earth, but an air molecule that is blown with the wind.
Is there a common name for such a coordinate system?
Firstly, I thought of it as the "Wind-Frame", but after some investigations I conclude, that the term "Wind-Frame" is meaning a different system which is also rotated and not only moved in the plane.
So if any body knows, I'd like to hear from you.
I use this because I calculate Dubins paths and want to take wind into account. So when projecting the path of the plane on earth, you will see a trochoid, but the plane itself will fly a plain circle in its surrounding air. This eases calculations a great deal.

Hello again. From outside this community, there was a proposal to call my reference frame "air-frame". (not to mix up with airframe).
What's your opinion?
Obviously, such a reference frame is hardly used as I didn't find anything in the literature at all)

Some people call it Amosphere-Fixed reference frame.
(see at: heli-air.net)
Just to add a thought, this name seems to imply a global reference system, whereas the velocity of air molecules may be very local.


text:
How do you calculate the lift coefficient of an airfoil at zero angle of attack?
Thin airfoil theory gives $C = C_o + 2\pi\alpha$,  where $C_o$ is the lift coefficient at $\alpha = 0$. However, I couldn't find any equation to calculate what $C_o$ is which must be some function of the airfoil shape. In other words how do you extend thin airfoil theory to cambered airfoils without having to use experimental data?
This is my own attempt, I made this airfoil model of the lift coefficient of the airfoil at zero angle of attack for a project I am working on. It's derived from a Joukowsky transform. it seems to work but how is $C_o$ actually calculated?

Because it is derived from the Joukowsky transform of the inviscid potential flow around a cylinder it's more accurate at high Reynolds numbers.

I marked this as "unclear what you're asking", but only for lack of a better label.  The problem, in just my opinion, with this question is that the only possible answer is 'yes' because we cannot disprove a negative.  Ie. if you've discovered a new way, then no one can realistically be expected to comprehensively prove no one has ever done it before.

The linear term ($C_0=4\pi \cdot p$) can be found in many books, while the extra terms affect the result only in single digit percentages. This is the same order of magnitude as the error, as well as smaller than the magnitude of other factors you're not taking into account such as the chordwise position of max camber. That's probably a reason why the formula is not commonly shown the way you wrote it down: the same way we use $C_l=C_{l\alpha}\cdot \alpha$ instead of $C_l=C_{l\alpha}\cdot \sin(\alpha)$, "$1-\sqrt(4p^2+1)$" makes the calculation more complex without improving the accuracy.

I see you created two accounts. If you wish to join them, please follow the instructions here: https://aviation.stackexchange.com/help/merging-accounts

also, please consider typing the equations here, rather than posting an image, it makes answering and searching a bit easier

The solution and the approximate solution to zero-lift angle of attack from thin airfoil theory can be found in ESDU 98011. The derivation of an analytical solution can also be found in Anderson, Fundamentals of Aerodynamics. $C_{l_0}$ can then be found by multipling it with $2\pi$.
Edited to add classical thin-airfoil result:
$$C_{l_0}=2\int_0^\pi{\frac{dz}{dx}(cos\theta-1)d\theta}$$
$$x=\frac{1}{2}(1-cos\theta)$$
$x$ is the chord line coordinate from 0 to 1; $z$ is the mean camber line height at coordinate $x$, normalized to the chord length; $\frac{dz}{dx}$ is the curvature of the camber line.
Now you just have to numerically integrate to get the lift at zero incidence.

This could be improved by citing the relevant parts of those sources.

An approach for finding $C_l$ of a cambered wing is to look at the angle of the tail section, and take this as if it was the $\alpha$ of a flat plate

In this example the max camber is pretty far forward, which makes a good illustration of the principle. It works best if the outflow has had ample opportunity to follow the aft end of the profile.
As an aside, $2\pi \alpha$ in an approximation at small angles of $2\pi \cdot sin  (\alpha)$


text:
Lift distribution on biplanes
This Wikipedia page about biplanes says

in a wing of aspect ratio 6, and a wing separation distance of one chord length, the biplane configuration will only produce about 20 percent more lift than a single wing of the same planform.

My question is, does the wing of  biplane share the lift load 50/50 ?
Secondly, what is the formula for calculating the interference drag of a biplane?

If both wings have the same incidence and the wings are not staggered (= they sit at the same lengthwise location along the fuselage), then yes, their lift is split equally between both.
Most biplanes were (and sometimes still are) single- or two-seaters and in order to give the pilot good forward and upward visibility, the upper wing is shifted forward a bit (positive stagger). This will place it closer to the upwash (flow field ahead of the wing with increased angle of attack) of the bottom wing and in turn place the bottom wing closer to the downwash (flow field past the wing with reduced angle of attack) of the top wing. This effect becomes stronger with increasing angle of attack of the whole biplane, in line with the lift coefficient. With positive stagger, the upper, forward wing carries the majority of the load. How much that is depends obviously on the amount of stagger.
The formula for the interference drag is the Biot-Savart law which describes the effect of one lift-creating object on its surroundings. Since both wings interfere mutually, the solution is not straightforward but needs several steps:

First, the location of a lift-creating vortex and its two trailing vortices is defined. A wing should be split into several sections which all have their own vortex strength. The more sections, the more precise the result will be. For a biplane this needs to be done twice, of course.
Then a number of points where the local flow direction is prescribed must be defined. Whereas the vortex usually is located at the quarter of the wing chord, those points are best placed at three quarters of the wing chord; again spread over the span of the wings. They are called control points.
Now the influence of a particular vortex on a particular control point is described using the Biot-Savart law. After this has been done for all vortices and all control points, those influences define a matrix.
By solving the matrix equation for tangential flow at the control points, the strength of the single vortices in the influence of all other vortices can be calculated.

A more elaborate description of the process can be found in NASA Technical Note D-5335.

You may complete your (already quite good) answer by talking about negative stagger (such as for the beechcraft staggerwing

@ManuH: That is only the same in reverse. How boring. The NASA code works for all values of stagger.


text:
What is the function of the leading edge strip?
I understand the function of every wing part labelled above except the leading edge strip,what is its use in a fabric wing and will the wing function normally without it,if removed?

A wing's leading edge is a place where you really want to have a particular shape and a nice, smooth surface.  Stretched coverings (I've done this a fair amount with monokote and other heat-shrink coatings on model airplanes) suffer in this regard since, between ribs, the covering will bend inward and alter the shape of the leading edge (and, locally, the chord of the wing).  Also, since the covering is all one piece, the shape of the leading edge will also influence the shape of the rest airfoil as you go aft along the chord line.  Maintaining the shape that you actually want is really important (unless you want to make tubercule wings: https://secretofflight.wordpress.com/leading-edge-tubercles-2/) as that is the basis of your aircraft's performance and its ability to fly.  Having a solid leading edge strip does just this, ensuring that the leading edge of the wing has the desired shape and forms a smooth surface for the covering.
For a physical description of this, what we did (on model aircraft) to facilitate this was to take a very thin piece of balsa wood, maybe 1/32" thick, and gently bend it around the leading edge of our ribs.  As such, this sheet, which ran the length of the wing, would conform to the shape of the leading edge.  A picture of this from a wing I built is below (the leading edge is on the left hand side -- and just ignore the fact that the wing is twisted...).  We added some sheeting to the trailing edge of the wing for similar reasons (and to make it easier to cover).

The year prior to this, someone took a dowel and ran it the length of the wing at the leading edge instead of covering it with a sheet of balsa -- a little less effective at shaping the airfoil behind the leading edge, but much easier to implement.  However, a similar idea appears to have been implemented in the DH-60 Gipsy Moth.

In addition to aerodynamic shaping, the further you can extend a contiguous structure in the chordwise direction, the stronger your wing will be in torsion (generally speaking) -- this is a significant function of a "stressed skin" coating like you are talking about.  However, the addition of a structural element, such as a leading edge cap, that goes the length of the wing adds a non-negligible amount of torsional stiffness to the wing.
This part is more conjecture, but the leading edge of the wing will also receive a lot of wear, be it from FOD, bugs, or cleaning off bugs.  When I flew sailplanes, the leading edge of our wings would constantly be filled with bug carcasses and we'd be cleaning them off frequently.  To this end, I'd think that having a solid leading edge would be an easy way to preventing damage during the cleaning process.  However, sailplane pilots care about that more than powered aircraft, I think, so, unless you're looking at an old Schweizer 2-22 wing, this probably isn't too much of a concern.

To emphasize the importance of the leading wing, this crash https://en.wikipedia.org/wiki/Continental_Express_Flight_2574 occurred when the leading edge on one side of the horizontal stabilizer came off, making the aircraft unflyable!

To add to Marius's answer, in some aircraft the leading edge "strip" will be a thin sheet of plywood that starts on top at the front spar, wraps around the leading edge ribs, then ends at the bottom of the front spar.
It forms a "D" shape if you look at it from a wing tip and use your imagination.  The plywood is firmly attached (glue, nails) to the top and bottom of the front spar as well as the ribs.
It is very strong and strengthens the wing considerably - especially in preventing the wing from twisting in flight.

Good point about the contribution to torsional stiffness, and also just a physical description of the item.  I'll add that into my answer.


text:
Would this be an accurate method of fuel consumption comparison?
I'm looking to compare the fuel consumption of regional jets in the US and the stated efficiency of the Bombardier CS100. At the moment, I'm considering using data from the Bureau of Transportation Statistics' Form 41 - T100D and T2. The T100D set provides the number of flights, distance, and aircraft type for each route and aircraft type. The T2 form provides overall fuel use by each aircraft type per airline.
I was considering plotting the average stage length for an aircraft type against the fuel used per seat nautical mile to arrive at a measure of efficiency as a function of stage length. I wanted to ask if this approach is methodologically accurate since I'm not entirely sure average stage lengths tell the entire story of how the aircraft is operated. If there are other models that could be more representative, could you point me in their direction?
My ultimate goal is to identify whether the CS100 could be more efficient than current jets. I've been looking at literature that estimates fuel use as a function of stage length and payload, but haven't had much success.
Thanks for your time.

@ymb1: Thanks so much for the link. Seems like I missed it when I searched around. While the numbers are right there, I was hoping to confirm them by using published data. The Wiki link does give me exactly the figures I'm looking to calculate, though.


text:
If the operator is not the CAMO who is responsible?
If the operator does not have a  Continuing Airworthiness Management Organisation (CAMO) and it contracts out its CAMO functions, does the operator remain responsible for the aircraft or does the contracted CAMO relieve some of that responsibility?

Please clarify what CAMO is.

I am speculating here, so standing by to be corrected by those in knowledge of facts!
It might be a good idea to study applicable regulations as well as the CAMO (Continuing Airworthiness Management Organisation) agreement in question; however operational responsibility will surely lie with the operator.
Sense test: If an aircraft ends up as a smoking hole somewhere, it most likely won’t do for the operator to say „that’s tough, but it wasn’t us, it was just the CAMO“.

You can contract out the authority to perform certain functions.  You can never contract out 100% of the responsibility.
If something happens you can be sure the operator will be named in the inevitable lawsuit.


text:
What is the difference between fuel control unit and fuel flow meter (transmitter)?
Hello can someone explain the difference between the fcu and fuel flow transmitter and how they work together alongside the fuel pump?

If you illustrate your question with a diagram of the fuel system you are talking about, it will clarify and focus the question.  This will help people answer it. The terms you are using are so vague (and so varied across manufacturers) that they are unhelpful out of context.

The Fuel Control Unit controls the flow of fuel to a turbine, it is in effect the mechanism by which the throttle works.
A fuel flow meter is simply a device that measures fluid flow (in this case the flow of fuel) through a line. The location, quantity and operation of flow meters varies by aircraft and configuration. Broadly speaking they are used to calculate actual fuel burn as well as to make sure the proper amount of fuel is flowing to an engine.
The fuel pump is the device that drives fuel to the engine/FCU from the tank. Again systems vary heavily but broadly pumps are used to move fuel from the lowest point in the tanks to an engine that may be above that point as well as to build sufficient fuel pressure to allow injectors/delivery systems to work.
In the simplest of systems a fuel pump sends fuel over a fuel line to an FCU. Somewhere on that line is a flow meter so that you can measure fuel flow out of the tank, into the engine.

This is right.  One thing that I might add to differentiate the FCU from the FFD is that the Full Authority Digital Engine Controller (FADEC) only communicates with the FCU, not the FFD.  FCU takes orders from the FADEC and reports back its flowrate.  FFD reports directly and exclusively to the cockpit for flowrate indication to the pilots.  Part of the start procedure on many FADEC controlled engines is to wait for the FFD to indicate positive flow before moving throttle or condition lever from "fuel shutoff".


text:
What are the lateral limits of Class D airspace in the USA?
Class D airspace was normally 5sm (4.4nm) with extensions for approaches, when they are less than 2nm.  If any extension is over 2nm, then the extensions are normally Class E.  Many publications claim a 4nm Class D, but searching JO 7400.11A 1, there are many Class D airports with 4.2 or 4.4 or even greater lateral limits.
Some non-FAA publications claim that Class D is 4nm, but few Class D are in fact 4nm, most are larger, and many have extensions.  KFDK has a 5nm lateral limit for Class D, and Niagara Falls, KIAG has a 5nm lateral limit with an extension for approach.  Reading PA has a 4.2 and 4.8nm lateral limit depending on which side of the airport you are on.
My understanding that that Class D may be influenced by gradients for IFR operations, but also that Class D is frequently a customized airspace.
Furthermore, as an example, Class C airspace, when the radar is OTS (out of service) has the tower revert to Class D.  Will that be 4nm or 5sm?
Also, an operating federal control tower does not mean that an airport is Class D.  There are airports with towers which lack WX reporting, and are not Class D.  One which comes to mind in the past is LCQ, which is now KLCQ.
So where is the definitive set of rules which define Class D?
Where does a pilot obtain the lateral limits of Class D when Class C (or B) is not in place because of radar outage(s)?

Class D airspace is whatever is charted. If the tower is closed then the charted airspace reverts to Class E or G depending on the type of weather reporting that is available.

I don’t believe that Class C airspace ever reverts to Class D. Can you give an example of where this happens?

According to TRACON controllers, when radar is OTS, the tower airspace becomes Class D and the Class C airspace no longer exists, until radar services become available.  I have observed this practice at various Class C airports.

I believe that automated WX reporting maintains the airspace as Class E when the tower is OTS.

Not always, it depends on the type of ASOS. KSBP is Class D and reverts to Class G when the tower is closed. KSBA is Class C and reverts to Class E when the tower is closed. All of the airports that I frequent follow this pattern.

I have a hard time believing that airspace changes depending on radar coverage. If Class C changed to Class D that would mean that transponder rules and communication rules would also change. I’m pretty familiar with the FAA handbooks and the AIM and I’ve never seen that mentioned. Also never heard of it in the wild. It would be helpful if you gave a citation from the controllers manual.

Sorry I am not reading the controller's handbook.   I can tell you that the Class D reversion has happened at several Class C airports. Normally it is NOTAMed. When it happens unexpectedly, the NOTAM is on ATIS and the approach controllers and clearance people will advise.

@mongo in that case the NOTAM would indicate if the entirety of the Class C was treated as Class D, or if there were new lateral boundaries. So "the charts, as augmented by the NOTAMs" would be the succinct answer.

Charts get NOTAMs also.  I asked about the rules for defining Class D.

@JScarry I found one. Shaw AFB (KSSC) has both class C and D areas noted on the chart. It says to check notams/supplement for hours. But the supplement says class C is 1200-0430Z, other times reverts to class E. Don't find any notams on the subject. Talk about confusing

@TomMcW (KSSC) Interesting find. It appears that back in 2003 the tower was open continuously, so they needed a Class D when approach went home. Now that they have the same hours, KSSC looks like a normal Class C—i.e. it reverts to Class E when tower and approach are closed. https://www.federalregister.gov/documents/2002/12/02/02-30328/proposed-establishment-of-class-d-airspace-shaw-afb-sc

@JScarry at my tower when the radar is OTS we have this blurb laid out in our SOP, to be added to the ATIS: "Primary radar out-of-service; Class Charlie services unavailable. VFR services are available only to aircraft with operating transponders and are limited to traffic advisories and safety alerts." I don't think it's made explicit what airspace class is in effect. (Further complication: That was written when we only used a single radar site. Now we have STARS with its FUSION mode drawing from multiple sites and ADS-B so I'm not entirely clear on if we even have to use that blurb.)

JO 7400.2L, Procedures for Handling Airspace Matters, describes how the size of Class D airspace is determined. Per paragraph 17−2−5. DETERMINING CLASS D AREA
SIZE:
The size of a Class D area, and any necessary extensions, is determined
by the use of a 200 feet per NM climb gradient and information obtained
from the person responsible for developing instrument procedures (see
FIG 17−2−1).

The figure lays out the formula for determining the radius and gives an example. In short, it depends on the distance from the airport reference point (ARP) or geographic position (GP) to the end of the outermost runway, and the terrain surrounding the airport. This is why the difference varies at different airports.


text:
What material is used for the floor panels in the main cabin for Boeing 747-400?
Just conducting some research into materials used in construction for Boeing 747-400 and Douglas DC-3.
I just found out that in a DC-3 the floor panels in the main cabin and companionway are of plywood.
As such, I tried to find the information with regards to Boeing 747-400, but was unsuccessful.
I was hoping someone could provide me with information with regards to the material of the floor panels in the main cabin and companionway of Boeing 747-400.

This illustration is earlier than a -400. Later ones might differ. There are two types: the normal one has aluminum skins bonded to a PVC core, for galleys/hold it is titanium skin bonded to a PVC core with greater strength/corrosion resistance.

747 cutaway view

Later a honeycomb sandwich was used, apparently two cross plied glass-epoxy prepreg skins bonded to phenolic coated nomex honeycomb core using an epoxy film adjesive. From:

Handbook of Adhesives and Sealants: volume 1, 2005

Thank you. That's what I was looking for. I wonder if the honeycomb structure was introduced into 747-400.

I can't speak specifically of the 747, but most floors in airliners these days are carbon fibre sandwich with a nomex honeycomb core.  Older ones may be fibreglass/nomex.


text:
Must Part 91 pilots comply with runway departures marked as NA?
I answered a recent question about the meaning of Takeoff Minimum NA and indicated that they do not apply to Part 91 pilots, but after thinking about it, I’m not so sure. I can’t find anything specifically saying that you can’t use runways marked as NA, although often the reason that the runway is not available would make using it a very unwise decision. e.g. terrain or obstacles. On the other hand an unmonitored ground facility when the tower is closed wouldn’t be an issue if you were using GPS to fly the departure.
The AIM 5-2-8 just says,
Pilots operating under 14 CFR Part 91 are strongly encouraged to file and fly a DP at night, during marginal Visual Meteorological Conditions (VMC) and Instrument Meteorological Conditions (IMC), when one is available.
Part 91 §91.175   Takeoff and landing under IFR. (f) Civil airport takeoff minimums. excludes Part 91 operators by giving limitations that only apply to Part 121, 129, and 135 operators.
A a towered field with an operating control tower, they are unlikely to allow you to to takeoff under IFR on a runway marked as Takeoff Minimum NA but is there any restriction when the tower is closed or there is no tower?

No. Part 91 is pretty much the wild west. Weather is 100RVR and 0VV? Whatever...cleared for takeoff! Contact departure on one two zero point thr...wait, he disappeared from the scope.  Does anyone hear an ELT?

@acpilot We do have some limits. For example, we must file an alternate if the weather is below minimums, and the alternate must comply with the alternate minimums. (BTW I think you are correct, but I’d like to know for sure.)

I would not take off somewhere if conditions were too low to prevent an immediate return for an emergency situation.

Other than flying a published SID or ODP with charted takeoff minimums, there are no other takeoff minimum requirements for pilots operating under Part 91.
If a Part 91 pilot receives and accepts a published departure procedure from ATC then they must comply with any minimums published on the procedure.
For example, some SID's may show takeoff minimums with a runway listed as "NA" (the Aspen Seven Departure as KASE lists "Rwy 15: NA - Terrain"). If you accepted this SID then you wouldn't be able to use runway 15 even operating under Part 91.
But just because you can doesn't mean you should. Every pilot flying Part 91 should have a set of personal minimums.
Just a pilot interpreting the regulations though, not a lawyer :)